How can I back up the game? All I can see is a transfer option but I need a transfer code to do it.
I've tried switching apps before but it doesn't recognise my old account and starts a new one.
I'm too scared to go to support in case they ban me
You can do that with Google account. You need to download a clean apk file (from apkpure, for example) and let the game link your account to google play. Then when it does, you need to install the same clean apk on the other device (it also has to have the same system, so iOS to iOS or Android to Android), and let it link your google and download the game state. After that you can install the modded apk on both and have the same account on both devices. As long as you do something on device #1 and the game is closed on device #2, when you open the game on #2 all data should be synchronized with #1.
I did that yesterday, so I can confirm it works, though I do have a rooted device and that allowed me to easily install the modded app over the pure one.
EDIT: just saw your original question, about the purchases.If your device is rooted and you applied patches from Lucky Patcher, you should be able to just install a clean apkover the one you currently have. If you don't, you can try the Titanium Backup route, but I can't vouch it works...